Discussion of git log, with emphasis on reachability.

git log displays repository history based on reachability:

        o---o---o---o---o  master
             \       \
              o---o---o---o---o  next
                           \
                            o---o---o  topic

Identify the specific starting point(s):

$ git log master
$ git log topic master

Identify revisions not of interest:

$ git log next --not master
$ git log next ^master